I have been reading about the libraries people have written for languages like Java and C# that make use of byte code weaving to do things like intercept function calls, insert logging code, etc. I have also been reading up on Lisp/Clojure macros in an attempt to better understand how to utilize them. The more I read about macros, the more it seems like they provide the same kind of functionality as byte code weaving libraries. By functionality, I mean the ability to manipulate code at compile time.
Examples of libraries I have been looking at would be AspectJ, PostSharp, and Cecil.
Is there anything that can be done with one and not the other? Do they actually solve the same problems or am I comparing apples and oranges?
Best Answer
Byte code weaving and macros are two different things.
Byte code weaving is a way to intercept function calls, so that you can inject some sort of functionality (generally a cross-cutting concern such as logging) into a function call, either before or after the function is executed. Byte code weaving is done at the byte code level, which means that it occurs after compilation. The function itself is not affected. This is one of the techniques that Aspect Oriented Programming uses.
Macros are a way to extend the syntax of a language. In its simplest form, a macro is simply a way to record keystrokes, and then play them back using a hot key. Language macros work in a similar fashion; a keyword or other syntax construct substitutes for some sort of macro expansion. This is oversimplified, of course; a better example of a macro specific to Lisp can be found here.